1. Field of the Invention
The present invention relates to a liquid ejection apparatus and method, and more particularly, to a continuous type of liquid ejection apparatus and method.
2. Description of the Related Art
One of a continuous type of liquid ejection method involves continuously pressurizing liquid with a pump to push the liquid out from a nozzle, and vibrating the liquid with a vibration unit. In so doing, such a method creates a state wherein the liquid is regularly ejected from a nozzle as droplets. Since droplets are continuously ejected from a nozzle with this method, in the case of applying the method to an inkjet printing apparatus, it is necessary to sort the droplets used for printing (dot formation) from the droplets that are not used in accordance with data to be printed. With methods referred to as charge deflection methods, such sorting is conducted by selectively charging droplets, deflecting the droplets with an electric field, and causing the charged droplets to fly in a trajectory different from that of the non-charged droplets. Furthermore, among these methods, a method referred to as binary charge deflection method is provided with a charging electrode, a deflecting electrode, and a gutter along the droplet flight trajectory from a nozzle, such that non-charged droplets are used for printing, and charged droplets are captured and collected by the gutter.
Recently, significant improvements in printing speeds are being demanded, and for this reason improvements in droplet generation speed are being pursued along with improvements in drying speed after a droplet has landed on a print medium. For this reason, it is effective to cause droplets to be ejected and fly at high velocities, and also use highly viscous liquid (ink). Accordingly, an increase in the pressure applied to ink pushed out of a nozzle is sought. In the case of using a highly viscous ink, friction increases between the highly viscous ink and an inner wall of nozzle. This produces problems such as the following. If the pressure exerted on the ink is low, a liquid column cannot be formed instantaneously. Some of the ink stays near the nozzle outlet, which can grow to become a large ink buildup. If such ink buildup further grows in the case of a configuration that ejects ink downward, the ink buildup becomes unable to stay further in the nozzle and falls. The falling ink buildup may adhere to and stain the print medium, or it may adhere to the area around the nozzle outlet or the wall surface of a member forming the droplet flight channel and acts on droplets separated from a tip of the liquid column and influences their flight direction, which may impair print quality.
Consequently, in a continuous liquid ejection method, it is desirable to exert pressure when forming a liquid column such that the liquid ink is forcibly and instantaneously ejected from a nozzle, with this desire being stronger with higher liquid viscosities. This is because in the case where the pressure exerted on ink gradually changes until the desired ejection velocity is obtained, ejection becomes unstable in the initial stages, and problems like those described above occur.
Japanese Patent Laid-Open No. H08-258287 (1996) proposes the following technology with regard to not causing such an initial unstable state. A valve is provided between the interior space of a nozzle and an ink chamber. The valve is closed, before ink ejection is initiated, the interior space of the nozzle is emptied of ink, while the ink pressure of the ink chamber is increased such that the required ejection velocity is obtained when the valve opens and ink reaches the nozzle outlet. The value is then opened and ink is ejected in this state.
However, with the method of Japanese Patent Laid-Open No. H08-258287 (1996), ink contacts the inner wall of nozzle near where the ink is ejected from the nozzle, which causes lowered velocity. This problem of lowered velocity becomes particularly severe when using ink with a viscosity of 20 cP or more. With an ink that is not highly viscous, pressure can be corrected to compensate for the lowered velocity, but with highly viscous ink, the effects due to manufacturing inconsistencies in the inner wall of nozzle become greater, and there is an increased possibility that the ejection state will become unstable.